This invention relates to linear motor drives, and more particularly, to incremental or stepper drives in which a power transmitting belt or equivalent is advanced incrementally through precisely dimensioned successive steps by reciprocal dynamoelectric linear motor.
Stepper motors are well known and widely used in applications where a driven member is required either to be accurately positioned at specific increments or steps or to be synchronized for movement in steps corresponding to a related system or to a particular program. Such motors may be mechanical, electromagnetic or electromechanical in nature and, because of the almost unlimited switching capabilities of presently available electronic circuitry, the latter two classes of stepper motors may be operated at speeds normally associated with continuous motor drives. The potential for precision with stepper motors may be appreciated by consideration of the finite nature of a digital system with the relatively subjective nature of an analog system.
A primary advantage of a stepper motor in which electromagnetic force is used both to initiate and to stop a drive output movement through successive steps is that overall drive speed and control is limited only by the switching speed of electronic control circuitry and the inertia of moving parts. In this latter respect, the required inertia for any drive system makes it difficult to establish the precise increment through which a stepper motor output will position a driven member or element. This problem is traditionally addressed by the addition of mechanical pawl and ratchet mechanisms which are effective for their intended purpose but tend to limit drive speed in the overall system.
In a commonly assigned, copending application Ser. No. 45,822, filed June 5, 1979 and entitled "Linear Actuator", a dynamoelectric linear motor is disclosed in which an armature having a magnetically permeable core and a pair of permanent magnets of rare earth materials such as samarium cobalt, is received within a generally cylindrical bobbin supporting a pair of electrically conductive field coils appropriately positioned in relation to the armature such that alternate energization of the coils will effect reciprocation of the armature. By providing simple abutment or limit stops at the opposite ends of armature travel, the distance or throw of reciprocal travel is established precisely. Also, by providing an iron disc or washer in fixed relation to the bobbin, the stopped position or step at which the motor comes to rest is inherently fixed. Such motors additionally optimize the magnetic flux characteristics of permanent magnets formed of rare earth materials and enable extremely high speed operation at high efficiencies in a very compact, virtually miniaturized motor package.